Electrical transformers, such as those used in energy distribution networks can be subject to the unwanted injection of a direct current into the primary winding or secondary winding. The injection of a direct current of this kind, hereinafter also referred to as the DC component, can, for example, originate from electronic structural components, such as are used nowadays to control electric drives or even for power-factor compensation. Another cause could be so-called “geomagnetically induced currents” (GIC).
In the core of the transformer, a DC component results in a unidirectional flux fraction, which superimposes the alternating flux. This results in an asymmetrical control of the magnetic material in the core and is associated with a series of drawbacks. Even a direct current in the order of a few amperes can cause local heating in the transformer, which can impair the lifetime of the winding insulation. A further unwanted effect is increased noise emission during the operation of the transformer. This is in particular perceived as a nuisance if the transformer is installed in the vicinity of a residential area.
Various mechanisms that work actively and passively to reduce the operating noise of a transformer are known. For example, it is proposed in DE 40 21 860 C2 that noise emission be counteracted at its point of origin, namely that the magnetic action of the injection DC component should be controlled directly. To this end, an additional winding is attached to the transformer, a so-called compensation winding. This compensation winding, which usually has only a low number of turns, is fed with a compensation current, wherein the magnetic action of said compensation current is aligned such that it is directed opposite to the magnetic flux of the disruptive DC component in the core of the transformer. The injected direct current is set in accordance with an adjuster or a control device in conjunction with an assigned detecting element, for example a microphone. However, a measuring device of this kind does not meet the requirements for reliability and the desired lowest possible maintenance costs, which are nowadays imposed on transformers in an energy distribution network.
In order to detect the unidirectional flux fraction in the core of a transformer as reliably as possible, the unpublished PCT/EP2010/054857 suggests a sensor mechanism which operates as a kind of “magnetic bypass”: by means of a ferromagnetic shunt part, a portion of the main magnetic flux is branched off at the transformer core and fed downstream again. This branched-off flux component bypassing the core is used to determine the magnetic field strength in the core section bypassed by the shunt arm either directly or indirectly from a physical variable derived therefrom. This detection of the magnetic field strength, or magnetic excitation, is more reliable and more suitable for long-term use.
Known from WO 2004/013951 A2 is a semiconductor switching unit by means of which a compensation current is fed into a compensation winding of a transformer for purposes of DC minimization. A control device with an independent energy source sets a controllable frequency for the duration of the current flow of the semiconductor switch (MOSFET). In this context, the electrical energy for the generation of the compensation current is taken from a capacitor which is charged cyclically via the MOSFET free-wheeling circuit. However, in the case of transformers such as those used in an energy distribution network, a capacitor is not desirable as an energy store for reasons of reliability and due to the desire for low-maintenance long-term operation.